Vacuum cooling, preenfriamiento por vacío, lechuga, lettuce
The question “What are pre-cooling conditions for lettuce?”, posted at Research Gate, was answered by Ahmed Khedr, Damanhour University, including Dr. Saltveit, Department of Vegetable Crops University of California, Davis, CA, paper on lettuce (see the attached pdf), that explains that the preferred method for lettuce is vacuum cooling:
Vacuum-cooling is the preferred method for pre-cooling all lettuces (Hardenburg et al., 1986; Ryall and Lipton, 1979). For effective vacuum-cooling, containers and film wraps are perforated or readily permeable to water vapor. To aid cooling, clean water is sprinkled on the heads of lettuce prior to carton closure if they are dry and warmer than 25 °C (77 °F). A modification called hydro-vacuum reduces water loss during cooling.
Thorough pre-cooling is essential because mechanically refrigerated trucks do not have enough cooling capacity to cool warm lettuce during transit. Field heat retained in the densely packed cartons can be removed by forced-air where vacuum-cooling facilities are not available, but it is much less effective. Hydro-cooling is effective for non-heading lettuce types, but should not be used with head lettuce since the water retained in the head fosters decay.
The first image shows the vacuum cooling by Moelco Levante, recently installed in the company Urcisol, Aguilas, Spain. With a capacity for 30 pallets, it´s the largest in Europe.
The second image, by ecosecha.blogspot.com, shows the diversity of types of lettuce.